1. Field of the Invention
The present invention relates to a manufacturing method of a picture tube such as a cathode-ray tube (hereinafter to be called a CRT) to be used in a television receiver or as a computer display and the like.
2. Description of the Related Art
At first, relating to a general conventional structure of a picture tube, an example is described while referring to FIG. 1 showing a partially cut-away sectional plan view of a shadow mask color CRT having a rectangular screen.
In FIG. 1, reference numeral 1 denotes a CRT, which is basically formed as a vacuum glass bulb. This CRT 1 comprises a panel 2 formed of a panel screen 2A and a side face portion 2B surrounding periphery of the panel 2 and projecting to the rear side, being positioned at the front; a funnel 4 positioned in the middle being consecutive to the panel 2; and a neck 5 consecutive to the funnel 2; incorporating an electron gun (not shown), and positioned in the rear.
A fluorescent screen 3 is provided inside the panel screen 2A, and confronting this fluorescent screen 3, a shadow mask 6 having multiple holes is disposed. The fluorescent screen 3 must be composed of plural phosphors. In the manufacturing process, therefore, it is required to demount and to mount the shadow mask 6 to and from the panel 2 a plurality of times. Accordingly, the panel 2 and the funnel 4 are constituted to be separable, and the both members are sealed with frit glass, which is a glass solder. The sealed portion of the panel 2 and the funnel 4 is called a frit seal portion 7.
In the abovementioned general conventional CRT 1, since the inside is in vacuum state, flaws on the glass as the principal material may be expanded to cause cracks, which may possibly lead to breakdown called as an implosion. To prevent the implosion, usually, a cloth tape 8 is would around the panel side face portion 2B. The cloth tape 8 is further tightened with a metal ring 10 from above, and cracks formed at the panel screen 2A side are prevented from propagating into the frit seal portion 7 side to prevent breakdown, as an anti-implosion treatment. Besides, mounting lugs (not shown) for mounting the CRT 1 on the television receiver, are mounted simultaneously when fitting the metal ring 10.
FIG. 1, reference numeral 11 is an anode button sealed to the funnel 4, and 12 is a neck splice line which is a connection portion between the funnel 4 and the neck 5.
FIG. 2 is a schematic front view of the CRT 1. In FIG. 2, symbol O denotes a tube axis of the rectangular CRT 1, which coincides with a center Z of the fluorescent screen 3.
Incidentally, such CRT 1 is deformed in the ordinary design, by making vacuum, so that the panel screen 2A may draw a contour line nearly similar to the rectangular fluorescent screen 3. The broken line in FIG. 2 shows its contour line. It means that the principal stress (tension) of the outer surface of the bulb near the middle of each side of the rectangular fluorescent screen 3 (the direction along X, Y axis in screen 2A) as compared with the diagonal direction of the fluorescent screen 3.
FIG. 3 is a perspective view schematically showing only the fourth quadrant of the upper right quarter as seen from the front when the CRT 1 is in vacuum state. The hatching shows the portion with large principal stress in the outer surface. On the other hand, hatching in FIG. 4 shows the portion of large principal stress same as in the inner surface of the bulb. Its peak value is nearly same as in FIG. 3
In FIG. 3 and FIG. 4, the Y-axis section including the tube axis O (short axis section) is called as an S.A., the X-axis section (long axis section) is called as a an L.A., and the diagonal axis sect on is called as a D.A. As evident from FIG. 3, the X, Y axis end portions of the panel screen 2A, edge portion 2C, panel side face portion 2B, and vicinity of the frit seal portion 7 are problem points for the strength of the CRT 1.
FIG. 5 is a schematic diagram showing the shape of a mold match line portion showing the maximum shape portion as seen on the Z-axis section of the side face portion 2B of the panel 2. It is usually the portion to be tightened by the metal ring 10. In addition, FIG. 5 shows only the first quadrant (the upper right portion of the panel screen 2A of the CRT), same as in FIG. 3 and FIG. 4.
As shown in FIG. 5, the upper portion and lower portion of the panel 2 are formed in a relatively large radius of curvature RL, and the right and left side portions are formed in a radius of curvature RS nearly same as the radius of curvature RL of the upper portion and lower portion, while the corners are formed in a relatively small radius of curvature r as compared with the radius of curvature RL or RS. The radius of curvature r of the corners is smoothly connected with the radii of curvature RL and RS at both sides, and the contact point of the radii of curvature RS and r is D1, and the contact point of RL and r is D2.
In FIG. 5, the maximum positions of the panel side face portion 2B in the x, y axis direction are supposed to be xM, yM, respectively.
FIG. 6 is a schematic diagram showing the surface pressure received by the panel side face portion 2B when the panel side face portion 2B is tightened with the metal ring 10. The surface pressure is applied in the perpendicular direction to the side face portion 2B. Therefore in FIG. 6, the magnitude P of the surface pressure is indicated in the perpendicular direction of the panel side face portion 2B. The surface pressure between yM and D2 is constant at PL, the surface pressure between D1 and D2 is constant at PD, and the surface pressure between D1 and xM is constant at PS. In other words, between yM and d2, between D1 and D2, and between D1 and xM, the radius of curvature of the panel side face portion 2B is constant. Further, when tightened by the metal ring 10, the surface pressure is constant, individually. At this time, the surface pressure to the panel side face portion 2B is inversely proportional to the radius of curvature of the panel shape.
A practical example of surface pressure is shown in a 37-inch CRT.
xM=(391.8, 0), yM=(0, 309.0)
RL=5521.9 mm, RS=5433.8 mm, r=35.0 mm
PL=5.060.times.10.sup.-3 kgf/mm.sup.2,
PD=7.983.times.10.sup.-1 kgf/mm.sup.2,
PS=5.142.times.10.sup.-3 kgf/mm.sup.2
What is of note here is that RL, RS&gt;&gt;r, and therefore the effect of the metal ring 10 is almost dominant between D1 and D2 which are corners of the panel 2. This tendency holds true regardless of the CRT size, although the numerical values are slightly different.
FIG. 7 and FIG. 8 are graphs of the stress generated in the CRT 1 by the metal ring 10 shown in FIG. 6 expressed on the axis of abscissas taken from the center of the panel screen 2A to the neck 5 in the direction of the short axis section S.A., long axis section L.A. and diagonal axis section D.A., while the principal stress is plotted on the axis of ordinates. FIG. 7 shows the stress generated on the outer surface of the CRT 1, and FIG. 8 inner surface of the CRT 1. A characteristic fact herein is that the effect by tightening force of the metal ring 10 wound on the panel side face portion 2B mainly acts on the position of the metal ring 10, particularly effective on the panel side face portion 2B of the diagonal axis section D.A., while the effects on outside and inside are in a contradictory relation.
The panel 2 and funnel 4 used in the CRT 1 of the conventional glass bulb are generally made of distortion free or nearly distortion free glass bulb. In the CRT for projection television which is recently distributing widely, the load of the voltage x current inside the panel 2 is extremely large as compared with that of a CRT for general television receiver, and therefore the outer surface of the glass panel 2 is cooled in the state of use.
Besides, in order to further enhance the reliability, a reinforcing measure is taken, for example, by generating a compressive stress by so-called ion-exchange on the outer surface of the screen surface of the panel. FIG. 9 is a schematic diagram showing the state of compressive stress and tensile stress on the section of the glass when such measure is taken. As indicated by broken line, a compressive stress layer of about 20 .mu.m is formed on the surface side of the outside of the glass panel 2.
In this way, in the conventional CRT, in order to avoid the implosion, a glass bulb formed of a thick glass having enough strength was used, and it was heavy.